This invention concerns electronic circuitry which uses thin-film transistors (TFT's) and, in particular, circuitry which uses polysilicon TFT's in designs that are sensitive to the gate to source threshold voltage of the TFT's.
Display technology pervades all aspects of present day life, from televisions to automobile dashboards to laptop computers to wristwatches. At the present time, cathode-ray tubes (CRTs) dominate display applications in the 10-40 inch (diagonal) display size. CRTs, however, have many disadvantages including weight, lack of ruggedness, cost, and the need for very high driving voltages.
Recently, passive-matrix liquid-crystal displays (PMLCDs) and active-matrix liquid crystal displays (AMLCDs) have become dominant in midrange display applications because of their use in lap-top computers. For smaller pixel sizes and also for large projection displays, the AMLCD is becoming increasingly important. A major drawback of AMLCDs, however, is the requirement of a backlight, usually a fluorescent light source, that substantially increases the size and weight of the display. It also leads to reduced efficiency since the back illumination is applied continuously even for pixels in the off state.
Another type of display, an active matrix electroluminescent (AMEL) display, emits light by passing a current through a light emitting material. In the case of an AMEL display, an alternating current (AC) is passed through an inorganic light emitting material. The inorganic light emitting material is arranged such that dielectrics are present on either side of the emitting material. Relatively high voltages are required to generate sufficient light from the electroluminescent (EL) material. The relatively high voltages are typically between 100-200 volts.
Yet another type of active matrix display uses light emitting diodes (LED's) or organic light emitting diodes (OLED's). Only the devices that correspond to illuminated pixels are drawing power. Accordingly, displays which use LED's or OLED's may provide better power efficiency for equivalent illumination. Both LED's and OLED's are advantageously implemented in an active matrix configuration to form AMLED and AMOLED displays.
Each pixel in an active matrix display includes at least one and typically two thin film transistors (TFT's). The thin film transistor receives video data from a column driver on the display when the display row containing the pixel is selected. The TFT stores the received video data onto the capacitance of the pixel.
One material which may be used to form active matrix LCD displays is polysilicon. It is difficult to design peripheral circuitry, such as column drivers, with TFT's, however, because field effect transistors made from polysilicon exhibit different turn-on thresholds. The turn-on threshold of a metal oxide semiconductor (MOS) transistor is the gate to source voltage, VGS, needed to establish a conductive path between the source and the drain. Variation in the turn-on threshold for the polysilicon thin-film transistors is a problem which affects both the pixel structure and peripheral circuitry of AMEL, AMLED and AMOLED technologies. It also affects AMLCD displays in which a portion of the peripheral circuitry is implemented in thin-film transistors.
Displays which use OLED devices for the pixels, moreover, have another problem. OLED devices are current-mode devices, that is to say, the amount of light emitted by an OLED varies with the current that is conducted by the OLED. To form a gray-scale OLED display, it is desirable to carefully control the current that flows through each pixel. Polysilicon threshold variation complicates this task causing OLED pixel circuits to be designed with four or more transistors to minimize the effect of threshold variation and, so, improve the uniformity of the displayed images. Some of these pixel structures are described in copending patent application Ser. No. 09/064,696 entitled ACTIVE MATRIX LIGHT EMITTING DIODE PIXEL STRUCTURE AND CONCOMITANT METHOD.
The addition of transistors to the pixel structure, however, has a negative impact on device yield and also reduces the portion of the pixel area that emits light, reducing the brightness of the display.